1. Must have PHP5.2 or above 
2. Must have either APC extension or the PECL uploadprogress extension installed.
3. Test and figure out which extension is working for you - this is to be done outside of drupal. You can find the test cases here http://www.hyrme.com/node/18
4. Test that your upload's are enabled for Ajax. Not all filefields in drupal support AJAX uploads.
5. Both APC and uploadprogress require a few settings in your php.ini file:

   APC needs the following: 
     apc.rfc1867 = on
   and uploadprogress needs the following - replace the /tmp with a writable directory.
     uploadprogress.file.filename_template=/tmp/upt_%s.txt 

6. Install the uploadprogress module
7. Depending on which extension works for you enable the uploadprogress module and select the appropriate extension in the admin/settings/uploadprogress form.

and you should be ready to go.
